Vertebral compression fractures, as illustrated in FIG. 1, represent a generally common spinal injury and may result in prolonged disability. These fractures involve collapsing of one or more vertebral bodies 12 in the spine 10. Compression fractures of the spine usually occur in the lower vertebrae of the thoracic spine or the upper vertebra of the lumbar spine. They generally involve fracture of the anterior portion 18 of the affected vertebra 12 (as opposed to the posterior side 16). Spinal compression fractures can result in deformation of the normal alignment or curvature, e.g., lordosis, of vertebral bodies in the affected area of the spine. Spinal compression fractures and/or related spinal deformities can result, for example, from metastatic diseases of the spine, from trauma or can be associated with osteoporosis. Until recently, doctors were limited in how they could treat such compression fractures and related deformities. Pain medications, bed rest, bracing or invasive spinal surgery were the only options available.
More recently, minimally invasive surgical procedures for treating vertebral compression fractures have been developed. These procedures generally involve the use of a cannula or other access tool inserted into the posterior of the effected vertebral body through the pedicles. The most basic of these procedures is vertebroplasty, which literally means fixing the vertebral body, and may be done without first repositioning the bone.
Briefly, a cannula or special bone needle is passed slowly through the soft tissues of the back. Image guided x-ray, along with a small amount of x-ray dye, allows the position of the needle to be seen at all times. A small amount of polymethylmethacrylate (PMMA) or other orthopedic cement is pushed through the needle into the vertebral body. PMMA is a medical grade substance that has been used for many years in a variety of orthopedic procedures. Generally, the cement is mixed with an antibiotic to reduce the risk of infection, and a powder containing barium or tantalum, which allows it to be seen on the X-ray. Also, an iodine solution as an x-ray marker is often used in liquid form.
Vertebroplasty can be effective in the reduction or elimination of fracture pain, prevention of further collapse, and a return to mobility in patients. However, this procedure may not reposition the fractured bone and therefore may not address the problem of spinal deformity due to the fracture. It generally is not performed except in situations where the kyphosis between adjacent vertebral bodies in the effected area is less than 10 percent. Moreover, this procedure requires high-pressure cement injection using low-viscosity cement, and may lead to cement leaks in 30-80% of procedures, according to recent studies. In most cases, the cement leakage does no harm. In rare cases, however, polymethymethacrylate or other cement leaks into the spinal canal or the perivertebral venous system and causes pulmonary embolism, resulting in death of the patient.
More advanced treatments for vertebral compression fractures generally involve two phases: (1) reposition, or restoration of the original height of the vertebral body and consequent lordotic correction of the spinal curvature; and (2) augmentation, or addition of material to support or strengthen the fractured bone.
One such treatment, balloon kyphoplasty (Kyphon, Inc.), is illustrated in FIGS. 2A-D. A catheter having an expandable balloon tip is inserted through a cannula, sheath or other introducer into a central portion of a fractured vertebral body comprising relatively soft cancellous bone surrounded by fractured cortical bone (FIG. 2A). Kyphoplasty then achieves the reconstruction of the lordosis, or normal curvature, by inflating the balloon, which expands within the vertebral body restoring it to its original height (FIG. 2B). The balloon is removed, leaving a void within the vertebral body, and PMMA or other filler material is then injected through the cannula into the void (FIG. 2C) as described above with respect to vertebroplasty. The cannula is removed and the cement cures to augment, fill or fix the bone (FIG. 2D).
Disadvantages of this procedure include the high cost, the repositioning of the endplates of the vertebral body may be lost after the removal of the balloon catheter, and the possible perforation of the vertebral endplates during the procedure. As with vertebroplasty, perhaps the most feared, albeit remote, complications related to kyphoplasty are related to leakage of bone cement. For example, a neurologic deficit may occur through leakage of bone cement into the spinal canal. Such a cement leak may occur through the low resistance veins of the vertebral body or through a crack in the bone which has not been appreciated previously. Other complications include; additional adjacent level vertebral fractures, infection and cement embolization. Cement embolization occurs by a similar mechanism to a cement leak. The cement may be forced into the low resistance venous system and travel to the lungs or brain resulting in a pulmonary embolism or stroke. Additional details regarding balloon kyphoplasty may be found, for example, in U.S. Pat. Nos. 6,423,083, 6,248,110, and 6,235,043 to Riley et al., each of which is incorporated by reference herein in its entirety.
Another approach for treating vertebral compression fractures is the Optimesh system (Spineology, Inc., Stillwater, Minn.), which provides minimally invasive delivery of a cement or allograft or autograft bone using an expandable mesh graft balloon, or containment device, within the involved vertebral body. The balloon graft remains inside the vertebral body after its inflation, which prevents an intraoperative loss of reposition, such as can occur during a kyphoplasty procedure when the balloon is withdrawn. One drawback of this system, however, is that the mesh implant is not well integrated in the vertebral body. This can lead to relative motion between the implant and vertebral body, and consequently to a postoperative loss of reposition. Additional details regarding this procedure may be found, for example, in published U.S. Patent Publication Number 20040073308, which is incorporated by reference herein in its entirety.
Still another procedure used in the treatment of vertebral compression fractures is an inflatable polymer augmentation mass known as a SKy Bone Expander. This device can be expanded up to a pre-designed size and Cubic or Trapezoid configuration in a controlled manner. Like the Kyphon balloon, once optimal vertebra height and void are achieved, the SKy Bone Expander is removed and PMMA cement or other filler is injected into the void. This procedure therefore entails many of the same drawbacks and deficiencies described above with respect to kyphoplasty.
A wide variety of other instruments and methods are known for the repositioning of vertebral bodies to correct deformations in alignment or spinal curvature of the spine that may result from vertebral compression fractures or other disorders. Such instruments and methods can generally involve the use of bone screws, also referred to as bone anchors, that may be implanted in to vertebrae. Once implanted, the bone screws may be used to mount a suitable spinal fixation instrumentation, such as clamps, rods, or plates. Such spinal instrumentation can then be used, to achieve and maintain correction of the spinal deformity and to stabilize the repositioned vertebrae while the vertebrae fuse together. For example, referring to FIGS. 3A-D, various methods 30A, 30B, 30C and 30D can be used to apply forces (as shown by small arrows 32) to reposition fractured or displaced vertebrae, e.g., fractured vertebrae 35 and displaced vertebrae 36. Bone screws 38, rods 39 or other apparatus may be used to apply such forces 32 and to maintain proper alignment of the spine 34.
FIGS. 4-9 show examples of such various methods and apparatus that have been employed in the art to correct spinal deformities. For example, FIG. 4 shows an example of a typical pedicle screw and rod system 40 that may be used to reposition and stabilize vertebral bodies adjacent to a fractured vertebra. Referring to FIG. 5, an early effort at spinal reduction was a system 50 having threaded shafts to draw the vertebrae into proper alignment. Such an apparatus for use in straightening a spinal column by reducing displacement between adjacent vertebrae is disclosed, for example, in U.S. Pat. No. 4,611,581 to Steffee, the entirety of which is incorporated herein by reference.
In other systems, a separate reduction mechanism grasps the head of a bone screw implanted in a misaligned vertebral body. In such systems, the bone screw is generally braced against a rod or other longitudinal support element, and the screw head may be pulled to realign the vertebra toward the rod. For example, FIG. 6 shows a method 60 of pulling a bone screw or other anchoring element toward a rod, or longitudinal member (Universal Spine System, Synthes, West Chester, Pa.). FIG. 7 shows a method and apparatus 70 for reducing spinal deformity using a cable and a cable tensioning system as disclosed in U.S. Pat. No. 5,782,831 to Sherman. FIG. 8 shows a spinal fixation apparatus 80 comprising a screw with openings in the head section for accepting a tension-stable fastening device that may be looped around the longitudinal support element, as disclosed in U.S. Pat. No. 6,325,802 to Frigg. FIG. 9 shows a bone screw and rod apparatus 90 using strings for rod movement and stabilization as disclosed in U.S. Pat. No. 6,802,844 to Ferree. Each of the forgoing references is incorporated herein by reference in its entirety.
A drawback of all of the above-described apparatus and systems is that the rod or longitudinal element is fully pulled into a clamping mechanism of the screw or anchoring element, and firmly engaged. Such an arrangement can be cumbersome and difficult to maneuver into an appropriate position for moving the vertebrae in the desired direction, particularly considering the relatively large size of the components of most of the above-described systems.
Accordingly, there remains a need in the art to provide safe and effective apparatus and methods for minimally invasive osteopathic augmentation and to reposition vertebral bodies and restore lordosis of the spine.